Referring to FIG. 1, the basic architecture of a conventional RF transmitter chain 1 includes a digital-to-analog converter (DAC) 2, a filter 3, a mixer (up-conversion to the transmitted frequency) 4, a gain control circuit (Variable Gain Amplifier or VGA) 5 and a power amplifier 6 having an output coupled to a transmit antenna 7. The VGA 5 is used to adjust the output power of the transmitter chain 1 to the desired level.
In the conventional transmitter chain 1 the current consumption is typically constant regardless of the value of the input signal, typically represented by In-phase (I) and Quadrature (Q) input signal channels. However, there should be no signal at the transmitter output if the IQ input signal is zero. This condition implies, ideally, that the transmitter current consumption could then also be zero. Prior to this invention, however, it was not possible to adjust the current consumption of the transmitter chain such that only a required amount of current is consumed with each IQ input value.
Said another way, a change in the transmitter bias signal, and hence a change in current consumption, was not possible with prior art transmitter topologies, since the change in the transmitter bias signal would result in an undesirable amplitude modulation of the transmitted carrier signal. More specifically, by changing the current consumption of an amplifier the gain of the amplifier is also changed. Therefore, if one attempted to change the bias to, and hence, the current consumption of the mixer 4, the gain would also change. The change in the amplifier gain resulted in the introduction of the undesirable amplitude modulation of the transmitter output signal.
Reference can be had to the following commonly assigned U.S. Patents for descriptions of various prior art VGA circuits and techniques, used in RF transmitters as well as in RF receivers: U.S. Pat. No. 5,548,616, L. Mucke et al.; U.S. Pat. No. 5,752,172, J. Matero; U.S. Pat. No. 5,752,170, P. Clifford; U.S. Pat. No. 5,884,149, M. Jaakola; U.S. Pat. No. 6,009,129, T. Kenney et al; U.S. Pat. No. 6,060,950, J. Groe; U.S. Pat. No. 6,167,273, G. Mandyam; U.S. Pat. No. 6,084,471, R. Ruth, Jr. et al.; U.S. Pat. No. 6,178,313 B1, P. Mages et al.; U.S. Pat. No. 6,317,589 B1, A. Nash and U.S. Pat. No. 6,370,358 B2, J. Liimatainen.